Flare stack combustion apparatus and method

ABSTRACT

High-pressure air is discharged in the form of jets moving at a high velocity from nozzles ( 32 ) mounted on a ring manifold ( 30 ) that encircles the flare stack ( 10 ) at a predetermined distance below the flare tip ( 12 ), the upper portion of the flare stack being surrounded by an exterior shield ( 50 ) that is provided with internal guide vanes ( 36 ) at the top portion and perforated with air passages ( 52 ) at the bottom portion. The high-velocity air movement in the annular space creates a low pressure zone that in turn induces a larger volume of air from the atmosphere to enter the annular space between the shield and stack where it rises to the flame zone ( 58 ), thereby lifting the flame and enhancing turbulent air, fuel and waste gas mixing in the flame zone to minimize or eliminate smoke.

FIELD OF INVENTION

This invention relates to the construction and operation of flaringstacks utilized to burn undesired by-product streams for release intothe atmosphere.

BACKGROUND OF THE INVENTION

The flaring or assisted open combustion of undesired process by-productstreams is commonly used to oxidize and convert toxic gases and vaporsto their less harmful combustion products for release into theenvironment. A mixture of the undesired product and a fuel are directedto the base of the flare stack to form a feedstream that rises to theflare tip or stack outlet where the mixture is ignited in the combustionzone to form the flare or flame. The efficient and complete combustionof the mixture is not always achieved. When the process is not properlymanaged, smoke is also produced by this process. Smoke can be anindicator that the combustion process is incomplete, and that the toxicor otherwise undesired process materials have not been converted to lessharmful forms. Smoke is also a visible constituent of air pollution, andits elimination or reduction is a consistent operational goal.

In order to reduce smoke production, the installation of air and steamsystems in conjunction with flaring stacks has been undertaken by theprior art. The low-pressure air assist system uses forced air to providethe air and fuel mixing required for smokeless operation. A fan,commonly installed in the bottom of the flare stack, provides thecombustion air required. Steam assisted flare systems use a steam ringand nozzles to inject steam into the combustion zone at the flare tipwhere air, steam and fuel gas are mixed together to produce a smokelessflame. In some systems of the prior art, a concentric banner or shieldis provided that surrounds the flare tip or outlet.

Steam and low-pressure air assists for flaring are in common use becauseboth systems are considered by the art to be generally effective andrelatively economical as compared to alternative means for disposing ofthe undesired by-products.

However, both of these prior art systems have various drawbacks anddeficiencies. The low-pressure air assists requires a significantcapital expenditure for at least one fan that must be dedicated to theflare stack. Continuous operation imposed a rigorous maintenanceschedule and even a back-up system in case of a breakdown or majorrepairs.

Steam assist systems can require sophisticated control devices, haverelatively high utility requirements and maintenance/replacementschedules.

Various methods and configurations of apparatus have been proposed inthe disclosures of the patent literature to improve the efficiency ofcombustion of flare stacks. For example, U.S. Pat. No. 5,788,477discloses a ring manifold fitted with an array of nozzles that can bedirected inwardly, upwardly and at an angle that is displaced laterallyto inject high pressure air jets above the outlet of the fuel gas at thetip of the stack. Similarly, in U.S. Pat. No. 4,652,232, a plurality ofnozzles are mounted on a high pressure fluid manifold that encircles thestack and emits the fluid at or somewhat above the level of the rim ofthe waste gas stack, and the nozzles are angled upwardly.

Other constructions are disclosed and discussed in U.S. Pat. No.4,019,852 as the background in the art.

In the constructions of the first two prior art patents, the nozzles ormanifolds containing the high pressure fluid outlets are in closeproximity to the flame, if not engulfed by the flame under foreseeableatmospheric conditions crosswinds. The maintenance and replacement costsassociated with these arrangements can be significant.

It is therefore a principal object of the present invention to providean apparatus and method for enhancing the complete combustion of flaregases that is less expensive to install, requires minimal maintenance,and is adaptable to the varying operating conditions found in industrialplant operations.

Another object of the invention is to provide a method and apparatusthat is readily adapted for use with existing flare stacks withoutsignificantly modifying the existing stack tip or outlet configurations.

SUMMARY OF THE INVENTION

The above objects and other advantages are realized by the method andapparatus of the invention that utilizes high-velocity jets or streamsof air in an annular space defined by the stack and a concentric shieldand that are moving in the direction of the flame to create a zone ofrapidly moving air that is at a lower pressure than that of thesurrounding atmospheric air mass. This low-pressure zone drawsatmospheric air into the annular space and creates a larger mass of airmoving in the direction of the combustion zone. This larger mass of airis directed into the flame combustion zone to assist the flare achievecomplete combustion of the feedstream.

The principal novel aspect of this invention is the use of air jets thatinduce large volumes of air from the environment to flow upwardly from alow pressure zone. The apparatus used consists of one or moredistribution ring manifolds and associated nozzles that are positioned apredetermined distance below the rim or tip of the stack, the nozzleoutlets aimed to direct the high pressure air jets upwardly toward thetip and the flame. The preferred high pressure operating range is fromabout 30 to 60 psig, and more preferably, from about 30 to 35 psig.

The distance below the rim that the nozzles are positioned can bedetermined empirically or by applying known methods and mathematicalmodels and equations. The position will optimize the zone of lowpressure to maximize the influx or flow of atmospheric air into theannular space defined by the stack and its shield to create a zone ofturbulent mixing of air, fuel and waste gas at, and in the vicinity ofthe flame.

The nozzles can also advantageously be angled from the vertical axis andin a direction that is also generally tangential to the adjacent flarestack surface. The effect of this nozzle positioning will be to at leastinitially creating a swirling or helically rising series of air jets inthe annular space between the shield and stack.

To further promote this helical movement, a plurality of vanes aremounted in the annular space, preferably attached to the shield's innersurface. The vanes are preferably curvilinear and extend from at leastthe to the region proximate the end of the stack to the vicinity of thehigh pressure nozzles. The effect of the vanes on the rising expandingair mass is to create and/or maintain turbulent flow patterns that willenhance the complete combustion of the waste and fuel gasses in theflame.

In a further preferred embodiment, a plurality of low pressure airstreams are directed generally upwardly and inwardly from nozzlespositioned around the periphery of the rim or open end of the stack. Thepreferred operating range for the low pressure nozzles if from about 5to 10 psig.

The apparatus and method can be advantageously utilized with existing ornewly constructed flare stacks fitted with shields having an upper rimthat terminates at the same elevation as the tip or rim of the stack, orthose shields that extend above the stack, and are either straight-sidedor tapered.

The method of the invention provides an economical solution for thesmokeless flaring of undesired gases from production and processingfacilities. The high-pressure air is provided by piping that extends upthe exterior of the flare stack to a high-pressure air distribution ringmanifold and jets surrounded by a shield. A zone of turbulence iscreated that is needed for smokeless operation.

The specific configuration of the apparatus used in the practice of theinvention varies according to the flare gas rate and the geometry of theflare tip or outlet. The invention makes economical the use ofhigh-pressure air. The volume of compressed air required is relativelysmall compared to the requirements for either low-pressure air or thesteam used in the systems of the prior art. Moreover, the piping andnozzles are not subjected to the adverse effects of steam.

BRIEF DESCRIPTION OF THE DRAWINGS

The apparatus and method of the invention will be described below and inreference to the appended drawings wherein like elements are referred toby the same numerals and in which

FIG. 1 is a top perspective view, partly in section, showing onepreferred embodiment of the invention;

FIG. 2 is a cross-sectional view of FIG. 1, taken along section line2-2;

FIG. 3 is a top plan view of the embodiment of FIG. 1;

FIG. 4 is a schematic side view, partially broken away, of anotherembodiment of the invention used with a flare tip of a different design;and

FIG. 5 is a schematic side view, partially broken away, of a furtherembodiment of the invention used with a flare tip of yet a differentdesign.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be further described with reference to FIG. 1, inwhich there is schematically illustrated the upper portion of a flarestack (10) terminating in outlet or tip (12) that is open to theatmosphere. The stack is provided with one or more igniters (14) whichare utilized in the conventional manner to ignite the combustiblefeedstream as it exits stack outlet (12). In this embodiment, aconcentric barrier or shield (50) is positioned about the upper endportion of the stack, with its upper end (54) at the same elevation asthe stack outlet (12). The shield (50) is of a generally cylindricalconfiguration and can be supported by a plurality of brackets (55) thatare attached to the outer wall of stack (10). The composition of thecombustible feedstream (16) and the specific configuration of the stack(10), outlet (12) and igniters can be of any configuration known to theprior art, or any new design developed in the future.

In the practice of the embodiment of the invention illustrated in FIG.1, a high-pressure manifold (30) encircles the exterior of the stack andis provided with a plurality of high-pressure nozzles (32) or otheroutlets, each of which produce a jet of air that is directed upwardly inthe direction of the stack outlet and flame. The manifold (30) is fed byhigh-pressure air conduit (34) that is in fluid communication with asteady source of high-pressure air. In a preferred embodiment, the airis delivered to the nozzles at a pressure of about 30 to 35 psig.

The high-pressure nozzles are positioned on the manifold (30) atpredetermined intervals based upon the geometry of the flare stack,flare tip and the composition of the combustible feedstream and itspressure.

As will be understood from FIGS. 1 and 2, the discharge of thepressurized air streams from nozzles (32) at a high-velocity creates alow-pressure zone below the nozzles as the air moves upwardly. Air isdrawn into the annular region (56) between the stack (10) and shield(50). This induced air flow provides a large volume of air that risestowards the flame and eventually mixes with the hot gases to enhance thecomplete combustion of the fuel gas and undesired chemical(s) in thefeedstream. The mixing is turbulent, which further enhances the completecombustion of the feedstream.

In order to assure a sufficient volume of atmospheric air flow from thearea around and below the high-pressure nozzles (32), the externalshield (50) is preferably provided with a plurality of spaced airpassages (52) about its lower perimeter. The size, number and spacing ofthe air passages is determined with respect to the air flow requirementsof a particular installation. If the manifold is of a size andconfiguration that impedes the flow of air into and through the annularspace between the stack and shield, then additional air passages inshield (52) are provided to assure a sufficient volume of air flow toconstitute the volume required to enhance turbulence and completecombustion at the flame zone (58).

It is desirable to optimize the atmospheric air flow into the annularspace based on the configuration of the installation with which theinvention is used. The volume of the annular space should not be sogreat as to reduce the flow rate of the air mass and its turbulence.

As will also be apparent to one of ordinary skill in the art, during theflaring the shield (52) is heated as a result of its proximity to theflame. One effect of the heating of the shield is to cause atmosphericair coming into contact with the surface of the shield to be heated andtherefore expand and naturally rise. Even without the presence of thepressurized high-velocity air injected by the arrangement of the presentinvention, a natural vertical convection, or chimney effect, is createdin the annular space between stack (10) and shield (50). In accordancewith the present invention, this effect is enhanced and magnified by thelarge volume of atmospheric air that is induced to enter thelow-pressure zone in the annular space from below and around the airjets. The increased volume of rising air is heated, causing furtherexpansion and turbulence to enhance combustion in the flame zone.

The shield (50) around the tip also serves to increase the turbulence inthe combustion zone due to the high temperature difference between themetal and the air. The low-pressure transfer in the reaction orcombustion zone promotes a smokeless reaction, and also controls thewind around the flame. The amount of compressed air used in the practiceof the invention is very small compared to the air induced from theatmosphere. The ratio of compressed air volume to atmospheric air drawninto the annular space can be up to 1:300, depending on theconfiguration of the ring and nozzles.

With continuing reference to FIGS. 1 and 2, a plurality of spaced vanesor baffles (36) are optionally provided to direct the air flow in theannular space between the stack (10) and shield (50). In the interest ofclarity, the number of vanes illustrated is limited in FIGS. 1-3. Thevanes can serve to provide a more uniform air distribution at the centerof the flame by moving the expanding air mass in a directed path throughthe annular space 56 into which the vanes project. In a preferredembodiment of the invention, vanes are attached to the shield flankingeach of the high-pressure nozzles and are inclined from the vertical atany angle comparable to the angle of the air jet emanating from theadjacent nozzle. Thus, in the embodiment illustrated, a total of sixteenvanes will be provided, two associated with each of the eighthigh-pressure air discharge nozzles. The vanes can be of a spiralconfiguration to direct the rising air mass toward the stack rim.

In a further preferred embodiment, a plurality of low-pressure windcontrol nozzles (40), fed by low pressure conduits (42), are spacedabout the periphery of the stack outlet (12). Nozzles (40) are suppliedby a source of low-pressure air at about 5 to 10 psig.

As shown in FIG. 1, the nozzles (40) are in fluid communication with thepressure reducing device (45) downstream on conduit (42). Alternatively,a separate low pressure manifold system (not shown) can be provided.Other alternative arrangements for the either/or both the high and lowpressurized air feed and distribution systems will be apparent to thoseof ordinary skill in the art.

The wind control nozzles function to minimize the effect of atmosphericcross winds that can disrupt the optimum combustion pattern of theflame; and to push the carbon dioxide combustion product away from theflame to prevent further undesired reactions. In a preferred embodiment,nozzles (40) have a diameter of about 0.0625 m/2 mm and are positionedat 90° intervals about the top of the stack. The low pressure nozzles(40) are directed at a 45° angle to the diameter line across the stackopening.

In the preferred embodiment described above, manifold (30) is fittedwith a plurality of high-pressure nozzles (32). In an alternativeembodiment, the tubular manifold (30) can be machined or otherwiseprovided with a plurality of directionally oriented outlets for thedischarge of the high-pressure air in place of nozzles (32). Theseoutlets preferably are at an angle of about 45° and emit the jets ofhigh pressure air in a direction that is tangential to the adjacentstack surface, i.e., the horizontal vector of the air jet is normal to adiameter passing through the outlet.

Two further embodiments of the invention are illustrated in FIGS. 4 and5, wherein the same arrangements of high-pressure nozzles are employed.In FIG. 4, the shield (50) has an upper end (50′) that is inwardlytapered, and terminates above the end of the stack (10). In FIG. 5, theshield (50) is cylindrical and also terminates above the stack. It willbe understood that changes in dimensions and in the relative spacing andconfiguration of the shield and stack may necessitate some changes inthe apparatus and operating conditions, all of which will be within thescope of the invention and the routine skill in the art.

EXAMPLE

A field test of the method of the invention was undertaken with a flarestack that was producing a significant amount of visible smoke due toinsufficient oxygen in the fuel/waste gas mixture. The following dataestablish the efficacy of the method. In this test the air requirementswere only 1.2 times the steam requirements, based on volume flow rates.The smoke intensity data is based on an industry-accepted standard forcomparative measurements. The units are:

MMSCFD—millions of standard cubic feet/day; and

SCFM—standard cubic feet/minute.

Gas Flow HP Air MMSCFD Jets # SCFM Smoke Intensity 1 0 0 100% 1 16 3540% 1 8 50 0% 2 8 55 0%

The results indicate the beneficial effect of increasing the flow rateby reducing the number of nozzles from 16 to 8. No observable reductionin smoke intensity was noted with the increase in flow rates from 70 to75. It will also be understood that an additional reduction in the flowrate could be undertake to determine the optimum conditions for thisparticular set of smoke/fuel/waste gas parameters.

The above describes the principal features of the invention. It does notlimit its application and, as will be apparent to one of ordinary skillin the art, the details of the construction will vary with the geometryof the flare tip and other parameters relating to operationalcharacteristics of the installation. Those skilled in the art willrecognize and be able to ascertain many equivalents to the specificembodiments of the invention that are described herein using no morethan routine experimentation. Such equivalents are intended to beencompassed in the scope of the claims that follow.

1. An apparatus for enhancing the complete combustion of an undesiredchemical and to thereby minimize the formation of smoke in the operationof a flaring stack, the flaring stack having an outlet for the dischargeof a flare feedstream that comprises a combustible mixture formed by theundesired chemical and a fuel gas, an igniter located proximate thestack outlet for producing a flame from said combustible mixture, and ashield that is positioned coaxially around the outside surface of thestack proximate the stack outlet, the apparatus comprising: a. aplurality of high pressure air jet nozzles spaced apart at predeterminedpositions below and around the exterior periphery of the flare stackoutlet in an annular space defined by the shield and stack, each of theair jet nozzles being directed toward the stack outlet and in thedirection of the feedstream's movement; and b. a source of high pressureair in fluid communication with the plurality of nozzles, whereby thedischarge of the air from the nozzles forms a plurality of high-velocityair jets to produce a moving air mass that draws additional atmosphericair into the mass of air moving toward the stack outlet to therebyenhance combustion of the flare feedstream; and c. a plurality of lowpressure wind control nozzles positioned around the periphery of thestack outlet and directed inwardly, wherein a low pressure curtain ofair is formed to flow inwardly and upwardly from the stack outlet at thebase of the flame to minimize the effect of atmospheric cross windsdisruptive to an optimum combustion patterns of the flame.
 2. Theapparatus of claim 1 which further includes a high pressure airmanifold, each of the high pressure nozzles being mounted on themanifold, the manifold being in fluid communication with the highpressure air source.
 3. The apparatus of claim 2, wherein the manifoldencircles the flare stack in the annular space between the shield andthe stack.
 4. The apparatus of claim 1, wherein each of the plurality ofnozzles is positioned to direct an air jet at an angle that is acute tothe longitudinal axis of the stack and tangential to the stack wall. 5.The apparatus of claim 1, wherein the high pressure air source is at apressure of about 30 to 35 psig.
 6. The apparatus of claim 1, whereinthe exterior shield is concentric with the flare stack throughout thelength of the shield.
 7. The apparatus of claim 6, wherein thedownstream portion of the shield is provided with a plurality of airinlet passages.
 8. The apparatus of claim 6 which further includes aplurality of air directing vanes extending generally parallel to theangle of the high pressure air jets in spaced relation around theperiphery of the stack.
 9. The apparatus of claim 8, wherein theplurality of vanes includes a pair of vanes extending from the surfaceof the shield and adjacent each nozzle.
 10. The apparatus of claim 1wherein the plurality of low pressure wind control nozzles positionedaround the periphery of the stack outlet are directed inwardly at anangle of about 45 degrees to a diameter extending through the controlnozzle.
 11. The apparatus of claim 7, wherein the shield extends to aposition above the stack outlet.
 12. The apparatus of claim 11, whereinthe upper portion of the shield tapers inwardly.
 13. The apparatus ofclaim 1, wherein the low pressure nozzles are supplied with air at apressure of about 5 to 10 psig.
 14. The apparatus of claim 1, whereinsaid coaxial shield includes an upper end that is planar with said stackoutlet.
 15. The apparatus of claim 1, wherein said coaxial shieldincludes an upper end that extends above said stack outlet.
 16. A methodof enhancing the complete combustion of an undesired chemical andminimizing the formation of smoke from an outlet of a flaring stackduring operation, the method comprising: a. providing a flare feedstreamformed from a combustible mixture of the undesired chemical and a fuelgas; b. discharging the flare feedstream from the outlet of a flarestack; c. igniting the flare feedstream to form a flame in a combustionzone; d. providing a plurality of high velocity air streams in the formof air jets spaced apart at predetermined positions below and around theexterior periphery of the flare stack outlet, each of the plurality ofair jets directed to move upwardly along the wall of the stack towardthe combustion zone to thereby create a low-pressure zone below the endof the outlet of the flare stack, wherein the air jets cause an influxof atmospheric air into the low pressure zone and turbulence in thecombustion zone to enhance combustion of the flare feedstream; and e.providing a plurality of inwardly directed, low pressure air streamsfrom a plurality of wind control nozzles proximate the periphery of theoutlet of the flare stack, wherein a low pressure curtain of air isformed to flow upwardly from the outlet at the base of the flame tominimize the effect of atmospheric cross-winds disruptive to an optimumcombustion patterns of the flame.
 17. The method of claim 16, whereineach of the plurality of air jets moves from a position below the outletof the flare stack.
 18. The method of claim 16 which includes thefurther step of providing an exterior concentric shield extending aroundand spaced apart from the periphery of the portion of the flare stackadjacent the outlet to thereby channel the atmospheric air upwardly withthe air jets.
 19. The method of claim 18, which includes the furtherstep of providing the concentric shield with a plurality of openingspositioned adjacent the downstream end and extending through the shield.20. The method of claim 18, wherein the concentric barrier extends to aposition above the stack outlet.
 21. The method of claim 16, whichincludes the further step of directing said plurality of inwardlydirected low pressure air streams at an angle of approximately 45degrees towards the stack outlet.